In communications systems it is always the goal to receive a signal in much the same form as it was transmitted. Noise and interference in a given communications channel work against that goal. With the advent of digital technology, tools such as forward error correction, error detection, and information compression allow communications over channel that might otherwise be considered too noisy for quality analog communications to take place. Another of the many benefits of digital communications is that messaging between communicating equipment can take place in many ways. That is, as one device communicates with another, information about signal quality, signal energy level, and timing, among other parameters, can be communicated between the devices to ensure a robust link. This messaging can take place without affecting the perceived signal quality.
There are several ways in which messaging can take place between communications equipment. A first method is to dedicate a separate frequency channel for messaging. This assumes that additional frequency spectrum is available, which is not always the case. A second method is to dedicate a time slot in a framed digital signal, or using a portion of the header of a digital signal frame for messaging. This so called common signaling channel (CSC) is in widespread use in telephony, particularly in digital switch interface protocols. CSC still has the advantage of not disturbing the information contained in the primary signal, and also does not require additional spectrum. However, CSC reduces the available bandwidth allocated to the primary information signal since some is siphoned off for the messaging component. Another method of signaling is the use of so called bit-robbing or bit-stealing methods. In bit-robbed messaging, the primary information signal is altered according to some prescribed format. A popular format in telephony is to use two bits of every nth, typically sixth, data byte sent for signaling. For .mu.255 pulse code modulation (PCM) digital telephony signals, where the least significant bit of every 6th PCM code word is used for signaling, bit robbing implies that the effective number of bits per sample is 7 and 5/6 bits instead of eight. So, in such a signaling format, bit robbing essentially trades signal integrity for a slightly smaller frame width.
However, by employing forward error correcting codes, the effect of bit robbing can be significantly reduced. Assuming that the hamming distance between code words is at least 2, then using one bit of the code word still leaves some correction ability. Of course, the code word must be received without any further corruption. Unfortunately, conditions may be such that further corruption in the received signal is unavoidable.
In many communications systems, and particularly in wireless systems such as digital cellular, as a signal weakens between a communicating devices, the amount of messaging between the devices will typically increase. This is done for reasons such as ensuring the quality of the link and to initiate handoff between cells. At the same time, given that the signals are weaker, they are susceptible to noise and interfering signals. This is the case when, for example, a cellular phone is at the edge of serving cell, especially if it is a boundary cell of the region. In which case, if messaging is done through a bit robbing scheme, the error correction may be compromised to a point where errors are unrecoverable because of the additive effect of bit robbing and errors resulting from noise and interference. However, in some cases, the information would be recoverable if either the messaging or the noise/interference were not present.
At the same time, more complex systems, such as digital cellular telephone systems, have much more sophisticated signaling protocols. In general, any control signal passed from one device to another is referred to as a message. These types of systems send messages regarding, for example, hand-off between cells, subscriber identification numbers, and signal level measurements, to name but a few. These messages may become quite long, and may exceed several hundred bits in length. It is important that such messages are received intact. If the simple scheme of bit robbing is employed, the messages, in effect, would not have any type of forward error correction.
Therefore, it would be preferable to avoid the intentional compromise of error correction by bit-robbed messaging, and instead perform messaging in an alternative way. Thus, there is a need, in a digital communications system, for a means by which to send messages between communicating devices without performing bit-robbing, and without using a dedicated messaging channel in either a dedicated frequency or a dedicated time slot, and which would allow the message data to be coded for error correction.